1. Field of the Invention
The present invention relates to an image reading device for reading a document placed on a document table with a color CCD sensor.
2. Description of the Related Art
In a color image reading device that is used as an input portion of a color printer or a terminal of a personal computer, even when an image in uniform density is read, an image information that is output from an image reading device may cause uneven color density depending on pixels due to the lack of optical uniformity as non-uniform illuminance of a light illuminating the document surface, variance of characteristic of a color CCD sensor, decrease in the lightness of the visual field of a lens when reading the light reflected on a document and converting into image data by a color CCD sensor.
In order to compensate an uneven illuminance caused by the optical factors mentioned above, so far a white shading correction plate, etc. provided at the side of a document table is read bad photoelectric converted into image data and the white reference is judged from this image data whenever reading a document and based on this white reference, the shading correction is executed to correct the document read data.
Further, in a color CCD sensor, when reading a color image by dividing it into plural colors, the read data may become uneven for each color because there is a difference in spectral sensitivity characteristics of R (Red), G (Green) and B (Blue) filters. Therefore, in order to compensate the sensitivity difference in R, G and B filters, the shading correction is so far executed so far based on a white reference and at the same time, the color balance correction is executed so as to improve color reproducibility as disclosed in Japanese Patent Publication No. 6-57050.
However, the above-mentioned shading correction and the color balance correction are so far executed by reading the reflected light on the shading correction plate provided at the side of a document table and using this read value as the white reference. Actually, however, the illuminance of the light irradiating the shading correction plate is not always agree with the illuminance of the light irradiating a document placed on a document table. Therefore, when the shading correction is executed based on the shading correction plate as the white reference, the color balance does not agree with that at the document table and a delicate color difference is produced when reproducing an image by a printer, etc. Further, the shading correction plate is actually not pure white and therefore, the sensitivity difference of the RGB filters cannot be corrected.
On the other hand, the optical system for irradiating a document generally moves for scanning on a rail mounted in the main body of an image reading device and therefore, the accuracy control of the rail is required in order to obtain an image of high reproducibility. However, the maintenance of highly accurate control is difficult and actually, the quantity of reflecting light from the document surface varies depending on the tilt of the rail in the sub-scanning direction and the uneven image density is generated from the fluctuation in the quantity of light and a good image reproducibility is prevented.
Accordingly, for an image reading device, it is desirable not only to execute the shading correction and the color balance correction of image data but also to correct a delicate color difference caused due to a difference in illuminance at the positions of the shading correction plate and the document table. It is further desirable to obtain a full color image of high color reproducibility by correcting the color balance so as not to produce uneven density in the sub-scanning direction when reproducing an image irrespective of distortion of the rail supporting the scanning movement of the optical system to irradiate a document.